The present invention relates to a water-borne coating composition to be applied to automotive bodies and the like and a method of forming a multilayer coating film on automotive bodies and the like, more particularly, to a water-borne coating composition and a method of forming a multilayer coating film both giving coating films excellent in smoothness and appearance.
In the art, coatings used for industrial purposes are so-called solvent-borne coatings and contain an organic solvent as a diluent. Thus, coatings contain an organic solvent or solvents in large amounts. In recent years, however, water-borne coatings in which organic solvent contained is reduced and water is used as a diluent have been developed from the environmental protection viewpoint.
As such water-borne coating, for example, in Japanese Kokai Publication Hei-07-53913, a water-borne coating composition is disclosed which comprises a resin obtained by at least partly neutralizing a polymer containing an amide group-containing ethylenically unsaturated monomer, an acidic group-containing ethylenically unsaturated monomer and a hydroxyl group-containing ethylenically unsaturated monomer and an aqueous dispersion of carboxyl group-containing acrylic resin particles.
Generally, the coating films obtained from the water-borne coating so far known have a problem in that they are poor in smoothness and appearance as compared with the conventional solvent-borne coating. In particular, in the case of the application to automotive bodies requiring excellent appearance, the coating films obtained from water-borne coatings containing a luster color pigment as a color component have a problem in that they are very poor in flip-flop property, smoothness and film appearance as compared with the solvent-borne coatings.
Accordingly, it is the object of the present invention to provide a water-borne coating composition excellent in smoothness and film appearance and, particularly when a luster color pigment is contained as a color component, excellent in flip-flop property.
The present invention provides a water-borne coating composition comprising
an emulsion resin obtained by emulsion polymerization of an xcex1,xcex2-ethylenically unsaturated monomer mixture comprising not less than 65% by weight of a (meth)acrylate ester whose ester-forming moiety contains 1 or 2 carbon atoms and having an acid value of 3 to 50
and a urethane compound represented by the general formula (1) or (2): 
xe2x80x83in formulas, R1, R2, R3, R4 and R5 may be the same or different and each represents a hydrocarbon group, R1 represents a hydrocarbon group which may optionally have a urethane bond, R3 represents a branched or secondary hydrocarbon group, n is a number not less than 2, j is a number not less than 1 in the general formula (1) or a number not less than 2 in the general formula (2) and k and m each is a number within the range of 1 to 500,
wherein the content of said urethane compound is 0.01 to 20% by weight on the solid basis relative to the resin solid in the coating composition.
The present invention also provides a water-borne coating composition comprising
a water-borne resin resulting from dissolving or dispersing a resin having an acid value of 10 to 100, a hydroxyl value of 30 to 200 and a weight average molecular weight of 4,000 to 2,000,000 in an aqueous medium by means of a neutralizing base
and a urethane compound represented by the general formula (1) or (2): 
xe2x80x83wherein the content of said urethane compound is 0.01 to 20% by weight on the solid basis relative to the resin solid in the coating composition. Referring to the general formulas (1) and (2), it is preferable that R2 and R5, which may be the same or different, each is an alkylene group containing 2 to 4 carbon atoms or a phenylethylene group and that R3 is a branched or secondary alkyl group containing 8 to 36 carbon atoms. The composition comprises a color component.
The composition comprises a polyether polyol having not less than 0.02, on average, of a primary hydroxyl group per molecule, a number average molecular weight of 300 to 3,000 and a water tolerance value of not less than 2.0. The polyether polyol preferably has at least one primary hydroxyl group per molecule and a hydroxyl value of 30 to 700. More preferably, it has at least 3 hydroxyl groups per molecule. The composition preferably comprises a polyester resin and/or an alkyd resin.
The present invention also provides a method of forming a multilayer coating film comprising: applying a water-borne base coating to an article to be coated and then applying a clear coating thereonto, followed by curing by heating,
wherein said water-borne base coating is the above-mentioned water-borne coating composition.
The water-borne base coating preferably has an application viscosity (viscosity in the step of application) at 25xc2x0 C. of 500 to 5,000 mPaxc2x7s as determined on a single cylindrical rotational viscometer at 6 rpm.
The present invention also provides a multilayer coating film obtainable by the method mentioned above.